Overlap of Cargo Adaptor Binding Sites on Myosin V Coordinates the Inheritance of Diverse Cargoes.

Abstract

Membrane-bound organelles are essential components of eukaryotic cells. Similar to chromosomes, specific mechanisms exist to ensure that partitioning of each organelle to the daughter cell occurs. Moreover, the proper amount of each organelle is distributed to a specific location in the daughter. The budding yeast Saccharomyces cerevisiae, provides a unique model to determine the mechanisms of organelle inheritance. Unlike other organisms, where organelle movement requires an interplay between two cytoskeletal systems, actin and microtubules, most cytoplasmic organelles in budding yeast move solely on actin. Moreover, virtually all of the cytoplasmic organelles are moved by a single myosin V, Myo2. Despite the fact that one motor is involved, the itinerary and ultimate location of each organelle is unique. These itineraries are regulated in part through specific cargo adaptor proteins that link Myo2 to each organelle. The existence of specific cargo adaptors, suggested the possibility that cargo adaptors provide the main regulation of cargo movement, while the Myo2 motor provides an inert platform for cargo binding. In further support of this postulate, the vacuole-specific adaptor Vac17, and the secretory vesicle-specific Rabs Ypt31/Ypt32, bind to distinct sites on Myo2. This suggested that each adaptor might bind a unique location on the Myo2 cargo binding domain. Here we demonstrate that Mmr1 is a cargo adaptor that links the mitochondria membrane to Myo2. Notably, the binding site for Mmr1 overlaps with the binding site for Vac17, a member of the cargo adaptor complex that links the vacuole membrane to Myo2. Importantly, we find that Vac17 and Mmr1 compete for binding at this site. This competition had the unexpected result of regulating the volume of vacuoles and mitochondria inherited by the daughter cell. We find that eight of the nine known Myo2 cargo adaptors overlap at either of two sites. Vac17 and Mmr1 overlap at one site, while Ypt11 and Kar9 bind subsets of residues that also bind Ypt31/Ypt32, Sec4 and Inp2. Thus, competition for access to Myo2 may be a common mechanism to coordinate the inheritance of diverse cargoes.